Hi everyone,

I’m hand-drawing my cross sections for county building permits, and I’d appreciate any feedback or advice you might have to offer. Please see the attached picture.

As I’ve mentioned in previous posts, I’ll be building an unvented roof over a cathedral ceiling in zone 5a. As drawn, my roof will have 4″ of recycled XPS above deck, with 1″ spray foam under and R30 fiber batts between the rafters.

Does anyone see anything problematic?

What about the space between the rafters at the eaves? Assuming the ceiling and rafter bays are well-sealed with foam, is the space an issue?

I’ve been wrestling with whether to use 2x10s or 2x12s. As far as my span goes, the 2x10s work, but with the addition of spray foam beneath the sheathing in order to get the right foam-to-fluffy ratio, I’ll need to compress the R30 batts to get them to fit, compromising their R value. My thought is that this would be better than having an air gap to deal with in the 2x12s, not to mention slightly more affordable and more green in that I’d be using less wood.

I’ll have the advisement of a local contractor when building, so certain details will come from him, but he’s not especially attuned to green building techniques (though he’s interested in building upon his current practices as our county will be adopting the 2018 IRC next year), so I’d like to be absolutely clear on these sorts of details prior to building. I would appreciate any advice you’re willing to give.

Thanks in advance,

Chris